Experiments were conducted to localize kallikrein along the rat nephron using radioimmunoassay. The highest activity per unit length was in the connecting tubule. A significant content was also found in distal convoluted tubules, initial collecting tubules, and cortical collecting ducts. These data provide information which will aid the rational design of in vitro perfusion experiments to test the role of the kallikrein-kinin system in the regulation of NaCl and water transport in the kidney.